The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find further application in quality control inspections, spectroscopy, and the like.
Conventionally, magnetic resonance systems generate a strong, temporally constant main magnetic field, commonly denoted Bo, in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object. Nuclear spins of the object then possess a macroscopic magnetic moment vector preferentially aligned with the direction of the main magnetic field. In a superconducting annular magnet, the B.sub.0 magnetic field is generated along the longitudinal axis of the cylindrical bore, which is assigned to be the z-axis. In an open system, the B.sub.0 magnetic field is oriented vertically between a pair of pole pieces, which is assigned to be the z-axis.
To generate a magnetic resonance signal, the polarized spin system is excited at resonance by applying a radio frequency (RF) magnetic field B.sub.1, with a vector component perpendicular to that of the B.sub.0 field. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis, the precessing magnetic moment generates a magnetic resonance signal at the Lamor frequency which is received by the same or another radio frequency coil in a reception mode.
Birdcage coils are often used to excite and/or receive magnetic resonance signals, especially in horizontal field or bore-type MRI systems, because of the good B.sub.1, uniformity over a large field of view. In bore-type systems, the axis of the birdcage coil is typically aligned with the z-axis and the resident resonant current that generates the circularly polarized B.sub.1 field is sampled as an orthogonal pair of transverse modes.
However, aligning the coil axis with the B.sub.0, field in a bore-type machine can be problematic. Often, patients experience feelings of claustrophobia during head imaging applications due to the proximity of the birdcage coil to the patient's face. In addition, aligning the coil axis with the horizontal B.sub.0 field hampers fMRI applications, which require additional space near the patient's face for devices to stimulate the visual senses.
The present invention contemplates a new and improved radio frequency birdcage coil which overcomes the above-referenced problems and others.